Error with print to pdf

Hello i have this error when i use printtopdf function in planning:
2016-12-20 14:14:51.083 ***** ERROR ***** EXCEPTION *****
2016-12-20 14:14:51.083 ***** ERROR ***** on file '/var/www/html/external/html2pdf/vendor/tecnickcom/tcpdf/tcpdf.php' at line (2927)
2016-12-20 14:14:51.083 ***** ERROR ***** cause = TCPDF ERROR: Some data has already been output, can't send PDF file
2016-12-20 14:14:51.083 ***** ERROR ***** => #0 /var/www/html/external/html2pdf/vendor/tecnickcom/tcpdf/tcpdf.php (7620) -> Error()
2016-12-20 14:14:51.083 ***** ERROR ***** => #1 /var/www/html/external/html2pdf/html2pdf.class.php (390) -> Output()
2016-12-20 14:14:51.083 ***** ERROR ***** => #2 /var/www/html/view/print.php (239) -> Output()
2016-12-20 14:14:51.083 ***** ERROR ***** => #3 /var/www/html/view/print.php (183) -> finalizePrint()

I have solve with this modification:
In /var/www/html/view/print.php i have add this line: ob_end_clean(); after line 238 ($html2pdf->writeHTML($html2pdf->getHtmlFromPage($content))

I would to export my gantt planning. Print gantt planning is very important for display in office or communicate by PDF with customer.